The measurement of spectroscopic, physical quantifies in the immediate vicinity of a sensor surface with the aid of optical surface waves is known in the art. Recently, applications of this methodology have been gaining importance in the area of bioanalytical chemistry and biosensing. These new optical analytical processes are based on the interaction of the surface wave with the molecules to be detected. The detection of substances on the sensor surface is carried out, for example, by measuring changes in the refractive index and/or the optical absorption.
Optical surface waves are guided light waves which propagate in or on a wave guide layer structure along a surface. Optical surface waves may be guided for example, with an optical wave guide layer. The wave guide layer consists of a thin, dielectric optical layer on a planar substrate. Alternatively, the wave guide layer structure may consist of a thin metal layer on a substrate. In this case, the so called surface plasmon propagating on the metal layer represents the surface wave.
High, surface-specific detection sensitivity may be obtained by using very thin wave guide layers with a very high refractive index. The thickness of the wave guide layer in this case is clearly below the wave length of the guided light wave. The refractive index of the wave guide layer should be as high as possible, typically higher than 2.
It is known in the art that in order for coherent light to be coupled into or decoupled from the wave guide layer, using one or more optical diffraction gratings, so called grating couplers, is known in the art. Use of a bidiffractive grating coupler is advantageous. The bidiffractive grating coupler causes a directional separation between the decoupled light to be detected and the reflected or transmitted portions of the incident light beam. This makes possible background free detection of the light of the surface wave after decoupling.
By measuring the effective refractive index of the guided surface wave, changes in the refractive index in the immediate vicinity of the wave guide layer may be detected with high sensitivity. Prior art processes for investigating the binding between so-called receptor molecules and molecules which bind specifically to the receptor molecules immobilized on the wave guide surface are based on this detection scheme. In this way, molecular interactions (for example, binding reactions, sorption processes) may be analyzed.
The determination of the effective refractive index of the guided surface wave by measuring the coupling angle is state of the art. Determination of the effective refractive index of the guided surface wave by measuring the coupling angle, poses high demands on the planarity of the wave guide layer. Furthermore, these prior art processes for detecting molecules on sensor surfaces pose high demands on the stability of the angle measurement and on the accuracy of the position of the planar wave guide relative to the measuring device. These great demands must be considered as a disadvantage in regard to wide use of the measuring method.
A process for manufacturing wave guide layers with high refractive index on planar plastic substrates has been described in the German patent application P 42 28 853.3. This process offers the advantage that an optical grating for coupling or decoupling of light may be manufactured in a cost-efficient manner by embossing the plastic substrate.